Image-forming apparatuses, e.g., copiers and printers, have in recent years been subjected to greater diversity in their intended uses and use environments as well as demands for greater speed, higher image quality, and greater stability. For example, printers, which in the past have been used mainly in the office, have also entered into use in severe environments, e.g., high temperatures, high humidities, and it is critical even in such instances that a stable image quality be provided.
Copiers and printers are also undergoing apparatus downsizing as well as advances in energy efficiency, and the use is preferred within this context of magnetic single-component developing systems that use a favorable magnetic toner.
In a magnetic single-component developing system, a magnetic toner layer is formed by a toner layer thickness control member (referred to herebelow as the developing blade) on a toner-bearing member (referred to herebelow as the developing sleeve) that is provided in its interior with a magnetic field-generating means such as a magnet roll. Development is carried out by transporting this magnetic toner layer to the developing zone using the developing sleeve.
Charge is imparted to the magnetic toner by the friction generated when the developing blade and the developing sleeve come into contact in the contact region between the developing blade and the developing sleeve (referred to herebelow as the blade nip region).
Reducing the diameter of the developing sleeve is a critical technology for reducing the size of the apparatus. With a reduced-diameter developing sleeve, the area of contact by the sleeve with the toner at the back of the sleeve is made small and as a consequence the charging opportunity is reduced. In addition, the developing zone at the developing nip region is narrowed and fly over by the magnetic toner from the developing sleeve is then impaired and the magnetic toner with a weak charging performance, i.e., a weak developing strength, will readily remain on the developing sleeve.
In this case, turn over of the magnetic toner in the magnetic toner layer within the blade nip deteriorates and charge rise by the magnetic toner is impaired.
In addition, when the diversification of the use environment is considered, it can be assumed that the magnetic toner will, for example, also undergo long-term standing in high-temperature, high-humidity environments. In such instances, the external additive attached to the magnetic toner surface undergoes a partial embedding due to softening by the resin component of the magnetic toner. When an extended durability test is carried out in this state, the external additive undergoes additional embedding due to the shear received by the magnetic toner in the blade nip region, and in the latter half of the extended durability test the flowability of the magnetic toner declines and charge rise is impeded.
In particular, with magnetic toners the dispersibility of the magnetic body readily exercises a substantial effect on the charging performance, as compared to magnetic body-free nonmagnetic toners, and various image defects are readily produced when the rise in the amount of charge on the magnetic toner is impeded.
To respond to this problem, numerous methods have been proposed in which the dielectric properties, which are an index for the state of the dispersion of the magnetic body within a magnetic toner, are controlled in order to bring about a stabilization of the changes in the developing performance that accompany changes in the environment.
For example, in Patent Document 1 the dielectric loss tangent (tan δ) in a high-temperature range and the normal temperature range is controlled in an attempt to reduce the variations in toner charging performance associated with variations in the environment.
While certain effects are in fact obtained under certain prescribed conditions, in particular adequate consideration is not given to a high degree of starting material dispersity for the case of a high magnetic body content, and there is still room for improvement with regard to the charge rising performance of magnetic toners and their fixing performance.
In order to suppress environmental variations by toners, Patent Document 2 discloses a toner for which the ratio between the saturation water content HL under low-temperature, low-humidity conditions and the saturation water content HH under high-temperature, high-humidity conditions is brought into a prescribed range.
This control of the water content does in fact provide certain effects for the image density reproducibility and transferability under certain prescribed conditions. However, no mention is made in particular of the charge rising performance and the fixing performance when the magnetic body is incorporated as a colorant in the reasonable amount, and this is inadequate for obtaining the effects of the present invention.
Patent Document 3 discloses an image-forming apparatus that contains toner particles as well as spherical particles that have a number-average particle diameter of from 50 nm to 300 nm, wherein the free ratio of these spherical particles is from 5 volume % to 40 volume %. This has a certain effect with regard to inhibiting, in a prescribed environment, contamination of the image carrier, scratching of the image carrier and intermediate transfer member, and image defects.
Patent Document 4, on the other hand, discloses a toner in which large-diameter particles are anchored and small-diameter particles are externally added. This supports an improvement in the fixing releasability and a stabilization of the toner flowability and makes it possible to obtain a pulverized toner with excellent charging, transport, and release properties.
Patent Document 5 discloses an art in which the coating state for an external additive is controlled and the dielectric properties of the toner are also controlled and that is effective mainly for the issue of streak prevention.
In these inventions, however, the free ratio of the spherical particles or large-diameter particles, as inferred from the anchoring conditions or free conditions of these particles, is relatively high, and control of the state of attachment of inorganic fine particles that are otherwise added is inadequate.
Due to this, the charge rising performance for magnetic toners is inadequate—for example, when an extended durability test is run after storage in a high-temperature, high-humidity environment, under which circumstances the state of attachment of inorganic fine particles is already susceptible to variation—and the effects pursued by the present invention are not obtained.
They are also inadequate with regard to control of the resin composition and/or viscosity and are thus unsatisfactory from the standpoint of securing the fixation temperature region intended for the present invention.
That is, there is still room for improvement with regard to obtaining a high quality image through a magnetic toner that regardless of the storage environment is capable of the long-term retention of an excellent charge rising performance and also has a broad fixation temperature region.